In a widely known image forming apparatus such as a copying machine, printer and facsimile, image forming is generally performed with the following procedure: The surface of a photosensitive member is exposed to a light image of an original in an optical system to form an electrostatic latent image on the photosensitive member. Then, electrically charged toner is adhered to tile electrostatic latent image in a developing unit to form a toner image and this toner image is transferred onto a paper sheet in a transferring unit. The toner image thus transferred is fixed on the paper sheet in a fixing unit. A conventional fixing device employed in such a fixing unit generally employs a heating means such as a heat roller or infrared lamp in order to heat toner thereby fusing tile toner on tile paper sheet.
In a fixing device using a heating means such as a heat roller, toner and a paper sheet are heated in the substantially same temperature environment and, therefore, there often arises the problem that the paper sheet is scorched or burnt. In order to avoid such a problem, a so-called flash lamp fixing device is recently often used, in which the luminous energy of a flash lamp is utilized. In a flash lamp fixing device, a flash lamp such as, for example, a xenon lamp flashes intermittently, causing toner on a paper sheet to instantaneously absorb its luminous energy so that the toner permeates the paper sheet with the resinous components being melted, and in this way, the toner can be fixed on the paper sheet.
The problem with the above flash lamp fixing device is that when fixing is carried out only by intermittent flashing of a flash lamp, luminous energy (PD.sub.p (V)) generated by one flash of the flash lamp is extremely small as shown in FIG. 23 and therefore toner cannot be fixed over a large area on the paper sheet.
For the purpose of effectively fixing toner in a specified area of a paper sheet, one proposal is disclosed in Japanese Patent Publication No. 62-40714 (1987). According to the flash lamp fixing device taught by the above publication, there are provided a reflector 53 enclosing a flash lamp 51 and a glass plate 54 disposed between the flash lamp 51 and a paper sheet 52, as shown in FIG. 24. The reflector 53 is made up of inclined face parts 53a which are inclined like an unfolded fan widened towards the paper sheet 52 and a back face part 53b disposed between the inclined face parts 53a and behind the flash lamp 51. The provision of the reflector 53 for reflecting part of light emitted from the flash lamp 51 towards the paper sheet 52 enables it to entirely increase luminous energy which reaches the paper sheet 52 by one flash, so that fixing can be performed on a larger area, as shown in FIG. 25.